The children's room is too warm in the new building.

I don't know how long your sessions in the guest bathroom are, but I think 21 degrees is warm enough and I wouldn't worry about that room.

But is it energetically good if the valve is on around the clock so that it sometimes doesn't even reach 21 degrees there?

Yes, you may have a short circuit in the circuit if it is very short. Slightly throttling certainly won't hurt, the temperature in the room is unlikely to suffer. Otherwise, if you can cope with the temperature, consider the room as "secondary". It will be as warm as it gets.

The alternative would be to increase the flow temperature just because of the room. The surrounding rooms or all rooms should basically be heated because heat migrates from one room to another.

Now first check if the circuit is fully open. A short circuit on the gas heating is not that bad, since the flow temperature is already low anyway. If the circuit is already fully open, still lower the flow temperature again and see how low it goes. I think there is still quite a bit of room. And the flow rate is really important, otherwise the gas heating won't be able to get rid of the heat and will cycle. We will do this step by step together now.

Now first check if the circuit is fully open. A short circuit in the gas heating is not that bad because the flow temperature is already low anyway. If the circuit is already fully open, still lower the flow temperature once more and see how low it goes. I believe there is still quite some room. And the flow rate is really important, otherwise the gas heating won't be able to get rid of the heat and will cycle. We will do this step by step together.

That is really kind of you and I am also very grateful for the many infos from you. But I think the flow temperature will not go any lower because a minimum temperature of 25 degrees is set.

What is the current flow temperature? You can gladly approach 25 degrees with the current temperatures. Nothing bad will happen. Polar bears won't suddenly invade you overnight.

After I set the heating curve to 0.4 yesterday, the flow temperature fluctuates between 26 and 28. According to the graph, it should be 30.